The process for preparing flushed pigments involves the addition of a flushing vehicle, such as an oil or a solution of resins and other additives in an oil or organic solvent, to a water-precipitated organic pigment known as presscake. The direct transfer or "flushing" of the pigment particles from the aqueous phase to the oil or non-polar phase is effected by kneading the presscake and vehicle together in a flusher or heavy-duty mixer under heat and vacuum. For a review of organic pigments and pigment dispersion processes, see "The Printing Ink Manual" fourth edition, Ed. R. H. Leach et al.; Van Nostrand Reinhold Co., Ltd; England, 1988, pages 602 to 604; and Kirk Othmer's "Concise Encyclopedia of Chemical Technology" Ed. D. Eckroth et al.; John Wiley & Sons, 1985, pages 869-892.
A dispersing resin for lithographic ink pigments must exhibit excellent pigment wetting properties when mixed with the highly aqueous presscake during flushing. These pigment wetting properties provide rapid and thorough coverage of the pigment particles and concurrent displacement of water originally bound to or trapped in the particle aggregates. A resin with good wetting properties adheres strongly to primary particle units and coats them to provide a stearic barrier to reaggregation. The resulting dispersed pigment concentrate exhibits improved color strength, gloss and transparency, as well as reduced bronzing. Representative examples of conventional commercial flushing components used in producing lithographic inks include soluble dibasic acid modified rosin esters, rosin modified phenolic esters, hydrocarbon resins and various specialty alkyds. A resin used as a flushing vehicle for lithographic inks must generally be stable in storage to viscosity, color and solubility. When used as an ink and applied to paper, the finished ink must dry quickly to provide a durable, smooth and uniform film which is resistant to abrasion and chemicals.
It is known in the art that useful dispersants for lithographic inks may be produced from phenolic resins prepared by reacting rosin with phenols and formaldehyde to provide a phenol-rosin condensate containing reactive double bonds. The phenol resin condensate may be further reacted with alpha-beta unsaturated carbonyl compounds to produce Diels-Adler and adducts, which may be esterified with polyols to produce resins with a low residual acid number.
As explained in U.S. Pat. No. 5,021,538 to Crews, these polyol esters of the phenol resin condensates are highly crosslinked, non-uniform three dimensional molecules which do not exhibit highly polar surface structures necessary for enhanced pigment wetting. The Crews reference teaches that pigment wetting characteristics may be improved by attaching polar polyamide functional groups to the polyol esters of the phenol resin condensates.